The invention relates to a viscous coupling with a housing including a housing casing and covers attached to its ends and which extend radially relative to the rotational axis. A hub is received in the covers so as to be rotatable around the rotational axis. First annular plates include apertures, especially arranged so as to be distributed, and having side faces which are designed to be planar, with the plates, via one of their circumferential faces, being non-rotatingly associated with one of the two coupling parts, the housing or hub, and the plates are held at a distance relative to one another. Second annular plates, via one of their circumferential faces, are associated with the respective other coupling part, the hub or housing, so as to be non-rotating and movable along the rotational axis. At least one second plate is arranged between two spaced first plates. The second plates, starting from the circumferential face which does not serve to establish a non-rotating connection, are provided with circumferentially distributed slots which extend along part of the annular thickness and whose edges are formed in such a way that they all project axially from a planar side face of the sectors formed between the slots. Also, the first and second plates at least partially overlapping in the radial direction. A high-viscosity viscous fluid, especially silicone oil, at least partially fills the interior part of the housing which is not occupied by plates.
Viscous couplings are known, for example U.S. Pat. Nos. 4,989,687 and 5,041,065, where the movable plates are divided by slots into sectors. The slots include limiting edges which project towards one side from the planar faces. The plates, which are secured so as to be spaced, are designed to be planar and they include two planar faces. The fixed and spaced plates may, optionally, be provided with apertures distributed on their annular faces. When a speed differential exists, the unfixed plates axially move between the housing and the hub, in the teeth, in the direction of the adjoining fixed, stationary plates, with their edges projecting in the direction thereof. Such movement is caused by the resulting hydro-dynamic effect favored by the edges because, as described in the above-mentioned publications, they form a running-in funnel and above all it is caused by the pressure differential.
Such movement continues until the movable plates provided with slots and bent edges abut the adjoining fixed plates. This also occurs during the so-called hump mode in the process of which there occurs friction-locking between the axially movable plates and the stationary plates. If the speed differential increases considerably, the rise in temperature causes a considerable increase in the pressure differential and there occurs the so-called hump mode. Because of the full friction locking effect, the speed differential during the hump mode approaches approximately zero, so that the transferable torque is clearly increased. The friction locking effect, especially during the hump mode, causes individual plate particles to be abraded. The separated metal particles are absorbed by the viscous fluid, but have a damaging effect on the service life and on the effect of the viscous fluid. It has been found that, in use, the viscous fluid gels (silicone oil with a high viscosity of 5,000 to 300,000 cSt). This, in turn, limits the service life of the coupling.